As a method of detecting the defects of a circuit pattern formed on a wafer by comparative inspection of an image in a production process of a semiconductor device, JP-A No. 258703/1993, for example, describes a method of comparatively inspecting a pattern by a so-called SEM method wherein an electron beam focused into a spot is used for scanning. The feature of a SEM inspection apparatus is that the resolution thereof is higher than that of an optical inspection apparatus and it can detect electrical defects. However, since a SEM inspection apparatus is based on a method of focusing an electron beam into a spot and obtaining an image by two-dimensionally scanning the surface of a specimen, when a specimen is inspected with the apparatus, long scanning time is required and thus the apparatus has an essential drawback to the future increase of inspection speed.
Further, as an electron beam inspection method attempting to obtain a higher speed, JP-A No. 249393/1995, for example, describes an inspection apparatus of a projection type wherein a semiconductor wafer is irradiated with a rectangular electron beam and generated reflection electrons and secondary electrons are focused into an image with an electron lens. A projection type inspection apparatus can be expected to form an image at a higher speed than the SEM method since it can irradiate an object at a time with an electron beam of a higher current than the SEM method and can obtain an image in an integrated manner.
However, in the case of a projection type inspection apparatus, a problem is that the distribution of the emission angle of secondary electrons is wide at the time of imaging. The distribution of the emission angle of secondary electrons follows the cosine rule and hence most of the secondary electrons are emitted at a large angle on the basis of the direction of the normal to a wafer. When all of such secondary electrons are taken into an objective lens and focused into an image, a sufficient spatial resolution cannot be obtained due to the aberration of the objective lens. In order to obtain a sufficient spatial resolution of a 100 nm level, it is necessary to form an image while the secondary electrons used are limited to those emitted at angles within a small angle of aperture (0.1 rad for example) to the axial direction of a lens. Therefore, even though a high current electron beam is used for irradiation as an areal beam in order to form an image, the proportion of the secondary electrons capable of actually contributing to the imaging is low and hence a required S/N ratio of an image is hardly obtained. In the case of using reflection electrons too, the obtained emission amount is smaller by double digit in comparison with the electric current of the irradiation beam and it is difficult to obtain both high defect detection sensitivity and high speed inspection simultaneously with a conventional projection type inspection apparatus.
In the meantime, as a method of securing both high sensitivity defect detection and high speed detection, JP-A No. 108864/1999 discloses a projection type wafer inspection apparatus wherein electrons pulled back before they impinge with a specimen by field reversing immediately above a wafer (hereunder referred to as mirror electrons or mirror reflection electrons) are used as imaging electrons.
A mirror electron imaging type inspection apparatus has two main features which are different from the features of a conventional projection type inspection apparatus wherein secondary electrons and reflection electrons are focused into an image. The first feature thereof is that mirror electrons from a specimen do not have such a wide angle distribution as secondary electrons have and are emitted nearly straight above the surface of a specimen, and hence it is possible to take almost all of the electrons into an imaging lens system and increase the amount of image signals. The second feature thereof is that, in a region where incoming electrons are mirror-reflected immediately above a specimen, the kinetic energy of the electrons reduces considerably and the track changes in accordance with even a slight deviation of a surface, and hence the difference in image contrast between defective portions and normal portions increases. It means that the load for image processing reduces to the extent that the difference in image contrast between defective portions and normal portions increases in comparison with a secondary electron and reflection electron imaging type inspection apparatus that obtains an image of a high resolution and detects slight difference of the image. In addition to those features, in a mirror electron imaging type inspection apparatus, most of the irradiation electrons are reflected immediately above a wafer and hence basically they do not enter into the wafer. Electrons having slightly higher energy exist in an electron beam since the electron beam has an energy distribution and those electrons enter passing through a potential barrier. However, the value is several eV at most. That is, a mirror electron imaging type inspection apparatus can deal with even a specimen which has the fear of damage caused by an electron beam with a SEM inspection apparatus or a secondary electron imaging type inspection apparatus as an object of the inspection.
A mirror electron imaging type inspection apparatus sensitively detects potential change formed by the unevenness of a surface and can have good sensitivity also to electrical defects formed on a wafer in the same way as a SEM inspection apparatus. For example, when a defect of no electrical-conductivity exists, since the portion is electrically insulated, the electric potential on the surface of the portion can be differentiated from that of an electrically conductive normal portion by electrification, and the abnormality of the potential can be detected by using mirror electron imaging. However, in a mirror electron imaging type inspection apparatus, an irradiation electron beam is mostly repulsed immediately in front of a wafer by field reversing and hence it is impossible to control the electrification of a specimen with the irradiation electron beam. As a consequence, it becomes necessary to control the state of the electrification of the surface of a specimen before the irradiation of a primary electron beam (preliminary electrification) in order to obtain a stable inspection image. The preliminary electrification can be carried out by: a method of irradiating a specimen to be inspected with light including ultraviolet rays or an electron beam having energy enough to generate secondary electrons; a method of applying a prescribed potential to the surface of a specimen; or another method.
JP-A No. 14485/2004 describes a preliminary electrifier to electrify a wafer before inspection. The electrification potential formed on the surface of a wafer by applying preliminary electrification varies in accordance with the type of an insulator and a circuit pattern and, since electric charge escapes little by little, electrification potential decreases at a certain time constant. Such a time constant is sufficiently long in comparison with the time required for obtaining a mirror electron image but is insufficient in comparison with the inspection time of a whole wafer, and thus additional preliminary electrification is required during inspection.
In order to detect electrical defects and therefor control the electrification potential of a wafer by the preliminary irradiation of an electron beam, a control electrode is disposed immediately above the wafer. The control electrode in the case of JP-A No. 14485/2004 is configured so as to transmit an irradiation electron beam and apply electric potential immediately onto the wafer by using a grid-shaped electrode. The principle of the control of the electrification potential by a grid electrode is explained hereunder. When preliminary irradiation is applied, the value of the irradiation energy of an electron beam is set beforehand so that the secondary electron emission efficiency may be one or more. In the case of a general insulative film material for a semiconductor device, the value is about 500 V. The surface of an insulative film formed on a wafer is positively electrified gradually by the irradiation of an electron beam since the secondary electron emission efficiency thereof is larger than one. When a potential relatively positive to the potential of a wafer surface is applied to a control electrode, the generated secondary electrons are pulled toward the control electrode and hence the wafer surface is positively electrified gradually. When the electrification potential of the wafer surface equals to the potential of the control electrode, then the electric potential gradient between the control electrode and the wafer surface is leveled and hence the generated secondary electrons begin to return to the wafer surface. As a result, the positive electrification of the wafer surface is alleviated, the electric potential gradient between the control grid and the wafer surface reappears, and the secondary electrons are pulled toward the control electrode again. As a consequence, the electrification potential of the wafer surface balances with the potential of the control electrode at a nearly equal potential level.
When a potential relatively negative to the potential of a wafer surface is applied to a control electrode inversely, the generated secondary electrons are pushed back from the control electrode and return to the wafer surface and hence the effective secondary electron emission efficiency becomes lower than one. In consequence, the wafer surface is negatively electrified until the electric potential gradient between the control electrode and the wafer surface is leveled. By so doing, the electrification potential of a wafer surface is controlled with a control electrode.
When it is attempted to further increase inspection speed and improve defect detection accuracy in the inspection of a wafer pattern, conventional technologies have had the following problems.
In the case of a mirror electron imaging type inspection apparatus, when electrical defects in a wafer are electrified by preliminary irradiation, it is necessary that the electrification potential is uniform in the entire preliminary irradiation region. The reason is that, in a mirror electron imaging type inspection apparatus, since an image is formed by using the reflection of irradiation electrons at a certain potential plane of a wafer surface, if the electrification potential of the wafer varies even slightly, the distance between the potential plane at which the irradiation electron beam is reflected and the wafer surface also varies and, as a result, the imaging conditions vary and the contrast of the mirror electron image also varies. As a result of the present inventors' experiments, it has been found that the allowable variation of the electrification potential is about 0.5 V or less and such a uniform electrification potential distribution cannot be attained only by simply disposing a grid electrode and applying electron beam irradiation.
Further, there are some cases where a preliminary electrifier is applied also to a conventional SEM inspection apparatus. However, in the case of a SEM inspection apparatus, secondary electrons generated when irradiation electrons enter a wafer during inspection electrify the irradiation region again and the detection efficiency of the secondary electrons does not vary with the variation of specimen potential being about several volts, and hence the highly accurate uniformity by the preliminary electrification is scarcely required. Here, in the case of a mirror electron imaging type inspection apparatus, most part of the irradiation electron beam is repulsed immediately in front of a wafer by field reversing and hence it is impossible to control the electrification of a specimen with the irradiation electron beam.
Furthermore, in the case of the mirror electron imaging method, the required uniformity of the electrification of a specimen surface is as strict as in the range of about plus or minus 0.5 V and sufficient uniformity has not been obtained with a conventional preliminary electrification technology used for a SEM method. When a conventional preliminary electrification technology is applied to a mirror electron imaging type inspection apparatus, concretely it is estimated that the following problems arise.
In an electrification controller, a problem is that a grid electrode is used as the control electrode and the distribution of electrification potential becomes two-dimensionally uneven. In the case of applying electrification control by the irradiation of ultraviolet rays or an electron beam through a grid-shaped electrode, the irradiation is not applied to the portions of a wafer corresponding to the grid, and hence the supply of electric charge is insufficient and the portions where an intended electrification potential is not attained are undesirably formed. Electrification control by preliminary irradiation can be carried out at any time including before or during the inspection of a specimen to be inspected, but it goes without saying that such unevenness of electrification badly affects the inspection accuracy.
When preliminary irradiation is applied during inspection with a preliminary electrifier or the like, the unevenness of electrification increases particularly in the direction perpendicular to the wafer moving direction. A wafer moves relatively to a preliminary electrifier and the irradiation regions partially overlap with each other in the wafer moving direction in many cases. Therefore, the electrification state is equalized in the direction parallel with the moving direction and the unevenness is alleviated to some extent. However, with regard to the unevenness in the direction perpendicular to the moving direction, such equalization is not applied and hence the unevenness in electrification potential still remains. As stated above, a problem has been that an abnormal contrast caused by the unevenness of electrification potential in a mirror electron image is falsely counted as a defect in real inspection and correct inspection is hindered.